1. Field of Invention
This invention relates to division of an aerosol cloud formed by a nebulizer within an Evaporative Light Scattering Detector.
2. Background of the Invention
An Evaporative Light Scattering Detector (ELSD) is an analytical instrument for detecting and quantifying samples that have been separated by any of a variety of chromatographic methods. Such methods include but are not limited to High Performance Liquid Chromatography (HPLC), Supercritical Fluid Chromatography (SFC), and Gel Permeation Chromatography (GPC).
The simplest embodiment of an ELSD has a nebulizer, a heated zone or drift tube, a light source, and an amplifier, which converts scattered light into an electrical signal. In operation, the column effluent, which contains both the mobile phase and analyte, is first sent to the nebulizer. The nebulizer transforms the effluent into an aerosol cloud, and propels the cloud into the instrument. As the aerosol cloud enters the drift tube, which is heated, the more volatile mobile phase evaporates, leaving a cloud of analyte particles. These particles scatter light from the light source. The scattered light is amplified by a photo-multiplier tube, photo-diode, or similar device into a useable electrical signal.
This simplest embodiment, referred to as “full flow” in U.S. Pat. No. 6,629,605 and illustrated as FIG. 1 in same, has many limitations. Principally, it will only evaporate modest amounts of volatile mobile phases. While limited, full flow instruments are quite sensitive within their permissible operating range. The ALLTECH MODEL 500 is an example of such an instrument. To address the problem of limited evaporative power, several solutions have been tried.
One solution, available on instruments form SEDERE involves a nebulization chamber (spray chamber) placed between the nebulizer and drift tube. The nebulized effluent is divided in this chamber by impaction/condensation on the walls of the chamber. The chamber is geometrically constructed such that larger aerosol droplets hit the wall and run out a drain, while smaller aerosol droplets follow gas flow through the spray chamber and enter the drift tube. U.S. Pat. No. 6,229,605 refers to these instruments as “split-flow” designs. As pointed out in the above-cited patent, split-flow instruments accommodate high effluent flow rates and difficult to evaporate mobile phases, but they do not always pass on enough aerosol to maximize sensitivity.
A second solution is available from ALLTECH ASSOCIATES, as a MODEL 2000. This instrument has a splitter that can be turned on or off, as described in the above-cited patent. Turning the splitter on involves rotating a plate impactor perpendicular to aerosol flow. Large aerosol droplets hit the plate, condense, and exit a drain. Smaller aerosol droplets traverse the annular space between impactor and wall, and continue on to the drift tube. Turning the impactor plate parallel to the aerosol flow essentially removes it from the instrument, which then becomes “full flow”. Thus the instrument is easily converted from “split-flow” to “full flow” modes.
The above-described design has advantage over earlier art, but still has objectionable limitations. Namely, (1) it has no intermediate settings, and (2) it relies on mechanical means (motor, solenoid or the like) to move the impactor.
As a detector for chromatography, an ELSD may reasonably be expected to handle a wide variety of effluent flow rates and mobile phase compositions. An on/off design, as described in U.S. Pat. No. 6,229,605, handles the extremes adequately, but cannot be optimized for moderate flow rates, or moderately difficult to evaporate mobile phases. Also, motors, solenoids, shaft seals and linkages are possible sources of mechanical failure.